...and it's pretty scary:
Varenicline, meanwhile, already carries warnings, as a result of Health Canada advisories issued in 2008, 2009 and 2010, concerning potential serious neuropsychiatric adverse events such as depression, agitation, hostility, behavior changes, suicidal ideation and suicide.
All that plus:
The meta-analysis of data from 14 double-blind randomized clinical trials found that people taking varenicline had a 72% higher risk of adverse heart-related events than those on placebo.
So when your Doctor tries to prescribe you Champix (what Pfizer call this drug in the UK), ask him or her whether they really think it's safe.